If this was someone's first video, then forgive me for saying this: It stunk.
There are plenty of better examples of Sonic sprite movies at Newgrounds.com . MUCH better.
Anyone who has seen Dark Sonic in action would know that he'd probably beat the crap out of regular Sonic.
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that aside, The sprites used were so limited it was rather sad, Dark Sonic was really nothing more than a recolor of the regular Sonic used in this video, and the music REALLY did not fit the scene. ALso, they took too long in those charge-up scenes to actualy throw their attacks. During the time they took, the other Sonic could have swooped in, broke the concentration of the opposite with a few hits, thereby removing the threat of the attack being released. Also, there was no actual END to the video, they just kept fighting.

If this is someone's first video, the best I can say is that you have great potential for improvement. And if it isn't, then stop wasting your electricity making sprite movies.